1. Field of the Invention
The present invention is directed to an x-ray apparatus having an annual vacuum housing within which an electron beam is caused to follow an annular path and is deflected from the annular path onto a target, thereby causing x-rays to be emitted from the target in a beam which is movable around the vacuum housing.
2. Description of the Prior Art
X-ray tubes are known which have an annular vacuum housing, an electron beam source which emits an electron beam entering into the vacuum housing, means in the housing for accelerating the electron beam, first deflection means for deflecting the electron beam such that it passes through the vacuum housing on an annular path, a target from which x-radiation emanates during operation of the x-ray tube when the electron beam is incident thereon, and second deflection means for deflecting the electron beam such that it is incident on the target surface, preferably substantially perpendicularly, in a desired position. It is also known to employ such an x-ray tube, in a computed tomography apparatus also having a detector unit and collimator means that limits the x-rays emanating from the point of incidence on the target during operation of the x-ray tube as a fan-shaped x-ray beam is incident on the detector unit.
X-ray tubes of the type initially cited are particularly useful for computed tomography because the demands made in a computed tomography system on conventional x-ray tubes are so high that their service life is extremely short compared to other radiological applications of such tubes. Moreover, in the increasingly employed spiral scan mode, in a computed tomography apparatus having a conventional x-ray tube, the tube must be electrically supplied with wiper rings. Wiper rings are relatively complicated and expensive, and may cause considerable disturbances in the electronics of the computed tomography apparatus due to contacting problems at the wiper rings as a consequence of the high accelerating voltage of the x-ray tube. Lastly, the image quality is disadvantageously influenced as a consequence of the extrafocal radiation emanating from conventional x-ray tubes.
X-ray tubes of the type initially described are advantageous here because
a rotating anode is not required, and thus the problems associated with rotating anodes (durability of the bearing, running noises) are avoided; PA1 insulation problems cannot occur when the target material evaporates during operation since the target is located outside the electron beam source; PA1 wiper rings are not required for the spiral scan mode and the disturbances associated therewith are thus avoided; and PA1 deterioration of the image quality due to extrafocal radiation cannot occur.
European Application 0 455 177 discloses an x-ray tube of the types initially described wherein, in a first embodiment, the aforementioned second deflector means is formed by a plurality of electromagnetic deflector elements arranged at equal intervals along the circumference of the vacuum housing. By successively activating the electromagnets, the path of the electron beam is controlled such that the point of incidence of the electron beam on the target is displaced along the circumference of the target in a scan motion in the way required for computed tomography. As a consequence of the large number of electromagnets required as well as the associated control circuitry, this solution is complicated and expensive. According to another embodiment disclosed in European Application 0 455 177--a corresponding x-ray tube is also disclosed by German OS 41 03 588--, the second deflector means are formed by a single deflector element placed at the circumference of the vacuum housing, again an electromagnet, whereby the field strength of the magnetic field generated with the electromagnetic is controlled such that the point of incidence is displaced along the circumference of the target in the required way. In this case, it is difficult to control the field strength such that the motion of the point of incidence ensues synchronously with the motion of the detector unit, as is required for computed tomography.
British Specification 2 044 985 discloses an x-ray tube having an annular target wherein a selectively activated electrostatic deflector element, such as an electrode, out of a plurality of electrostatic deflector elements arranged at equal intervals along the circumference of the vacuum housing is employed as the second deflector means. By successively activating the deflector elements, the electron beam is deflected such that the point of incidence of the electron beam is displaced along the circumference of the target in a scan motion.
German OS 26 20 237 also discloses an x-ray tube with an annular target and having a plurality of cathodes arranged at equal intervals along the circumference of the annular target. By successively activating the cathodes, a subject to be examined is transirradiated with x-ray beams from different directions in the manner of a scan procedure.